1) Field of the Invention
The present invention relates to a technology for compensating wavelength dispersion in optical fiber transmission.
2) Description of the Related Art
Conventionally, in optical signal pulse transmission using an optical fiber, the transmission speed in the fiber varies according to the wavelength. Therefore, as the transmission distance extends, the signal pulse wave becomes blunt. This phenomenon is called wavelength dispersion, which significantly degrades the reception level. For example, in single mode fiber (SMF), wavelength dispersion of 15–16 ps/nm/km occurs near a wavelength of 1.55 micrometers. One of the ways of compensating such wavelength dispersion is to apply an inverse wavelength dispersion of the same amount.
Currently, a dispersion compensating fiber (DCF) is widely used for the dispersion compensation. The DCF is designed to cause dispersion (structure dispersion) reverse to material dispersion of fiber material through special refractive index profile, thereby causing an inverse dispersion characteristic with respect to normal SMF (dispersion compensation of five to ten folds with respect to SMF of the same length). The DCF is connected to a transmission path fiber, such as an SMF, at a relay station, to eventually eliminate the dispersion.
FIG. 12 is a graph for explaining wavelength dispersion compensation and residual dispersion. The horizontal axis represents wavelength λ (nanometer), and the vertical axis represents wavelength dispersion (ps/nm). An optical wavelength range called C-band is 1525 nanometers to 1565 nanometers (No. 1Ch to No. 40Ch (channels). A characteristic line 1201 of an optical fiber (SMF) in single mode, a characteristic line 1202 of a DCF, and a characteristic line 1203 after dispersion compensation of the SMF by using the DCF are shown. Dispersion compensation using the DCF has the dependence of a dispersion characteristic on wavelength, which is called a dispersion slope. This wavelength dependence differs between the DCF and the transmission path fiber. Dispersion compensation using the DCF is performed with reference to a center wavelength for use in the SMF (in FIG. 12, near 1545 nanometers).
At both ends (near 1525 nanometers and near 1565 nanometers) of the optical wavelength range of the wavelength band (C-band) used in optical transmission in wavelength division multiplexing (WDM), wavelength dispersion that cannot be compensated occurs (residual dispersion represented by a value of N). Also, with an optical network route being able to be reconstructed (being reconfigurable), when a different type of a transmission path fiber is used for each route or an end-to-end transmission distance is changed, this residual dispersion value is varied with time. Therefore, it would be difficult to perform dispersion compensation in a reconfigurable WDM network with a DCF whose compensation value is fixed. Thus, a unit that compensates wavelength dispersion of a plurality of channels in a tunable manner at a high speed is desired.
The structure that tunably provides wavelength dispersion can be implemented in a spatial light system, which can be designed at a high degree of flexibility. As a principle of occurrence of wavelength dispersion, there is a scheme of causing light to pass twice through light distributing elements (for example, diffraction gratings, prisms, virtually-imaged-phased-array (VIPA) plates) that provide different angular dispersion for each wavelength.
FIG. 13 is a schematic for illustrating a principle of dispersion compensation using diffraction gratings. A light through an optical fiber 1301 at an input side is output to two diffraction gratings 1303 and 1304 via a collimate lens 1302. The diffraction gratings 1303 and 1304 are placed in parallel so that their grating surfaces are opposed to each other. The light is dispersed at the diffraction grating 1303, with its light at a short wavelength side passing through an optical path A1 and its light at a long wavelength side passing through an optical path A2 to the diffraction grating 1304.
Light A1 at the short wavelength side and light A2 at the long wavelength side output from this diffraction grating 1304 become parallel, and is then converged by a collimate lens 1305, thereby causing an optical path length difference (A1–A2). This optical path length difference causes wavelength dispersion, which is input to an optical fiber 1306 at an output side. By adjusting a distance between the two diffraction gratings 1303 and 1304, the amount of the caused wavelength dispersion can be varied. Other than that, although not shown, inverse dispersion can be provided by adjusting arrangement of the two diffraction gratings. To get a large amount of wavelength dispersion compensation, it is necessary to use light distributing elements having a large dispersion angle and to make a distance between the light distributing elements long.
To make the amount of wavelength dispersion tunable, the distance between the two light distributing elements is changed. Other than that, the two light distributing elements are provided therebetween with a unit that adjusts the optical path length (optical-path-length adjusting unit) to make the amount of wavelength dispersion tunable. For example, a structure is assumed such that light passes through one light distributing element, the optical path length adjusting unit, and then the other light distributing element in this order. In one example, light is reflected on a light distributing element by using a mirror as this optical path length adjusting unit, thereby achieving compensation of wavelength dispersion with a single optical light distributing element (for example, see Japanese National Phase PCT Laid-Open Publication No. 2000-511655 and Japanese National Phase PCT Laid-Open Publication No. 2002-514323).
FIG. 14 is a perspective view of a tunable dispersion compensator according to a first example of the conventional technology. This tunable dispersion compensator 1400 includes an input/output optical fiber 1401, a collimate lens 1402, a focusing lens 1403, a VIPA plate 1404 as a light distributing element (for example, see Japanese National Phase PCT Laid-Open Publication No. 2000-511655), a focusing lens 1405, and a free surface mirror 1406. The amount of compensation of wavelength dispersion is determined by the shape at an incident point of light incident on the free surface mirror 1406. The free surface mirror 1406 has a mirror surface 1406a curved and continuously varied with its upper portion forming a concave surface and its lower portion gradually forming a convex surface. This free surface mirror 1406 is shifted perpendicularly to a light distributing direction of the VIPA plate 1404, thereby variably setting the amount of compensation.
In the tunable dispersion compensator 1400 shown in the first example of the conventional technology, periodicity occurs in the compensation characteristic from the light distribution characteristic of the VIPA plate 1404. For use in WDM transmission, a channel interval is designed according to this period. Therefore, since the amount of compensation for every channel has the same value, a structure for dispersion compensation has to be separately required for each channel.
FIG. 15 is a schematic of a structure for achieving dispersion compensation for a plurality of channels. This is an example structure when the tunable dispersion compensator 1400 shown in FIG. 14 is used. After the wavelength of each channel in WDM transmission is demultiplexed by a wavelength demultiplexer (DEMUX) 1501, tunable dispersion compensators (denoted as VIPA1 through VIPAn) are provided correspondingly to the number n of required channels (Ch) for output to reception side devices (Rx1 through Rxn) 1502. As shown in the drawing, to achieve dispersion compensation of the plurality of channels by using the tunable dispersion compensators 1400 of the first example of the conventional technology, light has to be dispersed in advance at channel intervals, and also a large number of tunable dispersion compensators 1400 corresponding to the channels have to be connected, thereby requiring high dispersion compensation cost.
To get around the problem, a channel-by-channel (Ch-by-Ch) tunable dispersion compensator that achieves dispersion compensation of a plurality of channels with a single module is desired.
FIG. 16 is a perspective diagram of a tunable dispersion compensator according to a second example of the conventional technology. The components identical to those shown in FIG. 14 are provided with the same signs. A tunable dispersion compensator 1600 has a Ch-by-Ch-support structure. Compared with the structure of FIG. 14, a transmission diffraction grating 1601 is provided at a stage subsequent to the VIPA plate 1404. A transmission diffraction grating 1601 wavelength-demultiplexes light at channel intervals in a direction (in the drawing, vertical direction) perpendicular to a light distributing direction (horizontal direction) of the VIPA plate 1404. Also, a non-flat surface mirror 1602 includes a plurality of mirrors 1602a through 1602n arranged in the vertical direction in the drawing. These plurality of mirrors 1602a through 1602n are provided correspondingly to the number of channels n, and are structured so that their shapes can be changed to a concave shape or a convex shape separately from each other for each channel (for example, see Japanese Patent Laid-Open Publication No. 2003-29168).
In the non-flat mirror 1602, the mirrors 1602a through 1602n are made of elastically deformable material. A plurality (for example, approximately three for forming a channel compensation profile) of small actuators not shown are connected to the back surfaces of the mirrors 1602a through 1602n, and then these actuators are set so as to be operated to cause the curved surfaces of the mirrors 1602a through 1602n to each have an arbitrary shape.
However, in the technology according to the second example of the conventional technology, the shapes of the mirrors 1602a through 1602n of the non-flat mirror 1602 are changed to make the amount of compensation tunable. Therefore, it is difficult to maintain the shapes of the mirrors 1602a through 1602n for a long time because the material forming the mirrors is fatigued, deteriorated with time, or the like. If the mirrors 1602a through 1602n are not deformed to have desired shapes, a group delay ripple or the like will occur, thereby degrading a reception level. Moreover, the states of deformation of the mirrors 1602a through 1602 have to be detected for all channels. Therefore, a special detecting mechanism has to be required, thereby increasing cost.
Furthermore, the conventional technology is structured such that the diffraction grating 1601 is used for angular dispersion by channel. Therefore, to suppress the occurrence of a group delay ripple or the like, a high degree of processing accuracy is required for finely (with an accuracy of nanometers or higher) forming the curved surfaces of the mirrors 1602a through 1602n. Alternatively, to ensure a sufficient accuracy, the distance between the mirrors 1602a through 1602n and the diffraction grating 1601 has to be set long. In this case, the module is disadvantageously large in size.
At this point, according to the technology of the first example of the conventional technology, the free surface mirror 1406 has a fixed shape, and therefore the problems of the second example of the conventional technology described above do not occur. In the technology of the first example of the conventional technology, however, there is a problem that the amount of wavelength dispersion compensation cannot be set for each channel separately. Moreover, to make the amount of compensation tunable, the entire free surface mirror 1406 has to be shifted, thereby making it impossible to quickly perform an operation when the amount of compensation is changed.